1. Field of Invention
The present invention relates generally to electronic elements and particularly to an improved design and method of manufacturing miniature electronic components including transformers and inductive devices (e.g., “choke coils”) without a bobbin or other forming component.
2. Description of Related Technology
As is well known in the art, inductive components are electronic devices which provide the property of inductance (i.e., storage of energy in a magnetic field) within an alternating current circuit. Inductors are one well-known type of inductive device, and are formed typically using one or more coils or windings which may or may not be wrapped around a magnetically permeable core. So-called “dual winding” inductors utilize two windings wrapped around a common core.
Transformers are another type of inductive component that are used to transfer energy from one alternating current (AC) circuit to another by magnetic coupling. Generally, transformers are formed by winding two or more wires around a ferrous core. One wire acts as a primary winding and conductively couples energy to and from a first circuit. Another wire, also wound around the core so as to be magnetically coupled with the first wire, acts as a secondary winding and conductively couples energy to and from a second circuit. AC energy applied to the primary windings causes AC energy in the secondary windings and vice versa. A transformer may be used to transform between voltage magnitudes and current magnitudes, to create a phase shift, and to transform between impedance levels.
Ferrite-cored inductors and transformers are commonly used in modern broadband telecommunications circuits to include ISDN (integrated services digital network) transceivers, DSL (digital subscriber line) modems and cable modems. These devices provide any number of functions including shielding, control of longitudinal inductance (leakage), and impedance matching and safety isolation between broadband communication devices and the communication lines to which they are connected. Ferrite-core inductive device technology is driven by the need to provide miniaturization while at the same time meeting performance specifications set by chip-set manufactures and standards bodies such as the ITU-T. For example, in DSL modems, microminiature transformers are desired that can allow a DSL signal to pass through while introducing a minimal THD (total harmonic distortion) over the DSL signal bandwidth. As another example, dual-winding inductors can be used in telephone line filters to provide shielding and high longitudinal inductance (high leakage).
“Shaped” Devices
A common prior art ferrite-cored inductive device is known as the EP-core device. EP and similar devices are well known in the prior art. For example, U.S. Pat. No. 5,489,884 to Heringer, et al. issued Feb. 6, 1996 and entitled “Inductive Electric Component” discloses an inductive electric component including a coil body having coil body flanges defining a winding space, and contact pin strips integrally formed onto the coil body flanges, the contact pin strips having extensions and having free ends with undercuts formed therein being limited outwardly by the extensions. U.S. Pat. No. 5,434,493 to Woody, et al. issued Jul. 18, 1995 and entitled “Fixed core inductive charger” discloses an EP-core device, as well as other shaped core devices, including EE and RS devices. Other similar well-know devices include inter alia so-called EF, ER, RM, and pot core devices. See, e.g., the pot core device is described in U.S. Pat. No. 5,952,907 to McWilliams, et al. issued Sep. 14, 1999 and entitled “Blind Hole Pot Core Transformer Device.”
FIG. 1 illustrates a representative prior art EP transformer arrangement, and illustrates certain aspects of the manufacturing process therefore. The EP core of the device 100 of FIG. 1 is formed from two EP-core half-pieces 104, 106, each having a truncated semi-circular channel 108 formed therein and a center post element 110, each also being formed from a magnetically permeable material such as a ferrous compound. As shown in FIG. 1, each of the EP-core half-pieces 104, 106 are mated to form an effectively continuous magnetically permeable “shell” around the windings 112, the latter which are wound around a spool-shaped bobbin 109 which is received on the center post element 110. The precision gap in ground on the ferrite post 110 can be engineered to adjust the transfer function of the transformer to meet certain design requirements. When the EP core device is assembled, the windings 112 wrapped around the bobbin 109 also become wrapped around the center post element 110. This causes magnetic flux to flow through the EP core pieces when an alternating current is applied to the windings. Once the device is assembled, the outer portion of the EP cores self-enclose the windings to provide a high degree of magnetic shielding. The ferrous material in the core is engineered to provide a given flux density over a specified frequency range and temperature range.
The bobbin 109 includes a terminal array 114 (aka “header”) generally mounted to the bottom of the device 100, with the windings 112 penetrating through the truncated portions 116 of the half-pieces 104, 106, the terminal array 114 being mated to a printed circuit board (PCB) or other assembly. Margin tape (not shown) may also be applied atop the outer portions of the outer winding 112 for additional electrical separation if desired.
For each core shape and size, various differing bobbins are available. The bobbins themselves (in addition to the other elements of the parent device) have many different characteristics; they can provide differing numbers of pins/terminations, different winding options, different final assembly techniques, surface mount versus through-hole mount, etc. For example, U.S. Pat. No. 6,587,023 to Miyazaki, et al. issued Jul. 1, 2003 and entitled “Electromagnetic Induction Device” discloses a flat bobbin with coaxially aligned through-holes. U.S. Pat. No. 5,350,980 to Dye, et al. issued Sep. 27, 1994 and entitled “Nonlinear Inductor with Magnetic Field Reduction” discloses, inter alia, a dumbbell-shaped ferrite bobbin carrying an inductive coil.
Magnet wire is commonly used to wind transformers and inductive devices (such as inductors and transformers, including the aforementioned EP-type device). Magnet wire is made of copper or other conductive material coated by a thin polymer insulating film or a combination of polymer films such as polyurethane, polyester, polyimide (aka “Kapton™”), and the like. The thickness and the composition of the film coating determine the dielectric strength capability of the wire. Magnet wire in the range of 31 to 42 AWG is most commonly used in microelectronic transformer applications, although other sizes may be used in certain applications.
The prior art EP and similar inductive devices described above have several shortcomings. A major difficulty with EP devices is the complexity of their manufacturing process, which gives rise to a higher cost. The use of a bobbin (also called a “form” or “former”) increases not only the cost, but size and complexity of the final device, since the bobbin is retained within the device upon completion of the manufacturing process. The bobbin consumes space within the device which could be used for other functionality, or conversely eliminated to give the final device a smaller size and/or footprint.
Also, the EP core half pieces themselves are relatively costly to mold and produce. For example, by the time the EP transformer is assembled and tested, its volume production cost is high (currently ranging from approximately $0.50 to −$0.70). It would be desirable to produce a device having performance characteristics at least equivalent to those of an EP transformer, but at a significantly lower cost.
Yet a further disability of “headered” shaped core devices such as that of FIG. 1 is the use of the header or terminal array 114 itself. This component adds additional cost and manufacturing steps, and at minimum increases the vertical profile of the device 100. In certain applications, it would be desirable to utilize a lower profile configuration without a header if possible.
Bonded Wire
Bonded wire is a well-established product/process that is used to produce so-called “air coils”. Air coils themselves are inductors, and are typically use in RFID tags, voice coils, sensors, and the like. The materials and manufacturing equipment for producing bonded wire are commercially available from a variety of sources known to the artisan of ordinary skill.
Bonded wire is essentially an enamel-coated wire having additional coating applied (by either the wire vendor or the device manufacturer) to the outer surfaces of the enamel. Generally, during winding, the bonded wire coating may be activated (normally by heat, although other types of processes including radiation flux, chemical agents, and so forth) to cause the coated wires to stick/bond together. This approach provides certain benefits and cost economies in the context of electronic component production.
Accordingly, there is a need for an improved electronic device, and a method of manufacturing the device, that does not require use of a bobbin or other form(er). Such an improved device would ideally utilize existing and well understood technologies in place of the bobbin/form(er) in order to simplify the manufacturing process and further reduce cost, yet still maintain the desirable electrical and physical properties of its bobbined counterpart while reducing the overall size and/or footprint of the device.
Furthermore, for certain applications, it would be highly desirable to obviate the header (terminal array) of the prior art from the shaped core device altogether.